Metal extrusion process



'i'jnited States Patent O METAL EXTRUSION PROCESS Harry W. Dodds, Bay Village, and Charles Baldwin Sawyer, Cleveland Heights, Ohio, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application May 7, 1953, Serial No. 353,637 3 Claims. (Cl. 207-) The present invention relates to a method for working high-melting, refractory metals such as beryllium, tantalum, titanium, molybdenum, uranium, thorium, zirconium and the like.

It is diificult to extrude or otherwise work some metals which are not ductile at room temperature. Generally, it is necessary to heat such metals nearly to or above the temperature at which rapid oxidation begins before deformation may be successfully carried out. At these temperatures the metal must be protected from oxidation by a suitable means such as the use of inert atmospheres during the working or by the use of a protective envelope formed from another metal.

The principal object of this invention is the provision of a method for deforming refractory metals whereby the refractory metal is caused to assume the contour of the forming die in minute detail.

A further object of the present invention is the provision of a method whereby refractory metals may be deformed to the contours of a forming die below those temperatures at which rapid oxidation occurs, allowing the deformation to be carried out without protection from the atmosphere.

Other and further objects apparent to those skilled in the art are hereinafter set forth and described.

Beryllium is an example of a refractory metal with limited ductility at room temperature. At temperatures below 800 C., the massive metal is protected from rapid oxidation by a thin, adherent film of beryllium oxide. However, beryllium cannot be successfully worked at these temperatures by extrusion or similar processes under the prior art. The pressures required to extrude beryllium through a forming die at lower temperatures have generally been so great that the developed friction rapidly wears away or breaks the forming die. In addi tion, the product of such a procedure is usually so broached or cracked that it is worthless. 7

At temperatures above 800 C., the thin beryllium oxide film on the surfaces of the metal no longer prevents the infusion of oxygen, and rapid deterioration of even the most massive metal occurs. Heretofore, beryllium has been worked at these higher temperatures by enclosing the metal in an envelope of soft iron or a similar material which serves to protect the metal from reaction with the oxygen and nitrogen in the air and to furnish lubrication. The iron envelope is extruded with the beryllium and forms an outer casing which must later be removed.

We have found that it is possible to successfully extrude bare beryllium without the use of a protecting envelope at temperatures between 300500 C. (the upper temperature limit is established by the softening temperature of die steels) by inserting a conical graphite washer between the beryllium and the forming die. This graphite cone is crushed as the extrusion begins and forms a thin lubricating layer on the beryllium as it passes through the forming die, eliminating galling and seizing of the beryllium on the die material and greatly reducing the pressures required for extrusion, thus in turn reducing die wear and breakage greatly. The product of this extrusion has an excellent surface, free of cracks and broaching.

Figure l is an elevation view of an apparatus in which the process of extrusion may be conducted. A portion of a press capable of exerting several hundred tons pressure is shown as the table 1 and movable head 2. The press parts are protected by platens 3 and 4. Platen 4 is provided with an opening through which the extruded metal may pass. A plunger 5 rests on platen 3 and is fitted loosely into a retaining die 6. Attached to platen 4 is an extrusion die 7. A graphite plug 8 and beryllium billet 9 machined to fit the graphite plug are placed in the retaining diebetween the plunger and extrusion die. The retaining die is surrounded by a heating unit 10. I

Figure 2 is the tensile strength curve obtained by plotting the tensile strength of beryllium in pounds per square inch at varying temperatures between 25 C. and 850 C.

Figure 3 is a graph of the elongation curve of beryllium between the temperatures of 25 C. and 850 C.

Figure 4 is the graph of the reduction curve of.

beryllium between the temperatures of 25 C and 850 C.

A large number of beryllium extrusions have been made employing this invention. The procedure usually used was as follows. The press was set up as illustrated in Figure 1, except that a sheet of asbestos has sometimes been used between the platen and the press to de' crease the heat radiation from the die. The shear type extrusion die has an opening which is exactly the diameter desired on the extruded rod with a clearance allowed in the channel as shown (about in the case of A" rod). This provides a guide for the extruded metal. The beryllium billet to be extruded is machined to fit the retaining die loosely and one end is machined so as to provide a tip which fits into a conical depression (having a included angle) in the graphite washer as shown. This billet may be either vacuum cast beryllium, or metal which has been consolidated by the techniques of powder metallurgy. A hole may be drilled into the tip of the billet to aid in the collapse of the tip and the start of the extrusion, but this is optional. The billet may also be coated with aquadag, molybdenum sulfide or other extrusion lubricants well known to the art. Preferably, the billet is pre-heated to the extrusion temperature (about 425450 C.) before being placed in the die which is also held at the temperature of extrusion by the heating unit 10. After the billet is in place, the platen of the press is brought up against the plunger and the extrusion begins. The pressure needed to start the extrusion is greater than that needed to sustain the extrusion. The end of the extrusion is characterized by a rapid rise in pressure as the plunger comes against the extrusion die.

A variety of sizes and shapes of beryllium rod have been extruded by this technique, including round rod from 0.250" to 1.50" in diameter, squares and a special shape with a cross-section equivalent to the space between three closely-packed cylinders. Reduction ratios as high as 12:1 have been successfully used, but a ratio of 5:1 or less is generally more acceptable as the extrusion pressures are correspondingly lower with less die breakage and wear. Extrusion pressures to be encountered with the process disclosed herein are best illustrated by specific example. When producing a 1.0" diameter rod from a 2.25" diameter billet (extrusion ratio 5.06:1) the starting pressure averages about 54 tons per square inch with a pressure of about 50 tons per square inch near the end of the extrusion. Similarly, when producing a 1.50

diameter rod from a 2.25" diameter billet (extrusion ratio 2.25: 1) the starting pressure averages about 39 tons per square inch, while the finishing pressure is about 32 tons per square inch. Correspondingly, increased reduction ratios result in increased extrusion pressure, While reduced ratios result in decreased pressures. The pressures encountered with this process at the temperatures indicated are well within the strength limits of the highspeed tool steels used for die materials.

We have found that the temperature range of 300 500 C. is critical in the working of this process with beryllium, although beryllium does not oxidize appreciably until temperatures above 800 C. have been reached. This is because beryllium has a ductility peak in this temperature range whereby the ductility of the metal, as measured by tensile elongation, is greater at 450 C. than it is at 950 C. In fact the ductility at this latter temperature is little better than that at room temperature. This phenomena can be clearly seen by the following data obtained by tensile testing beryllium at the indicated temperatures.

Elevated temperature properties of two lots f hot-pressed beryllium Temp, U. T. 8., Percent Percent U. T. 8., Percent Percent 0. 1,000 p. s. i. Elong. Cont. 1,000 p. s. i. Elong. Cont.

Both vacuum cast beryllium and beryllium consolidated by power metallurgy techniques as well as cast and powder compacted thorium have been extruded by this process. The extruded rod has a density of 1.85 to 1.86 g./cc. (density of beryllium with ordinary impurities) for beryllium and 11.6 to 11.7 g./cc. for thorium even if the starting metal has a somewhat lower density prior to extrusion.

Thus, this invention, besides facilitating the extrusion and eliminating the necessity of providing at protecting envelope, has the further advantage of providing an extruded metal of substantially maximum density.

An extrusion was made to test the efiicacy of the graphite plug. Standard conditions, as previously outlined, were maintained with an extrusion temperature of 425 C. For this test, the graphite lubricating cone was omitted. A pressure of 160 tons per square inch was re quired to start the extrusion and after about four inches of metal had been extruded, the pressure built up so that no more metal could be forced out.

It has also been found possible to substitute graphite powder for the machined graphite cone. In this case the powder is tamped around the nose of the billet (giving a graphite shape similar to the machined cone) and held in position with a rod extending through the extrusion die onto the nose of the billet. A force of about 1000 cant.

This process may be applied to the bare extrusion of other refractory metals, of which thorium is an example. With thorium the starting billet is usually machined from the cast material, although the billet may be powder consolidated by powder metallurgical techniques as was the case with beryllium. The procedure and equipment used in the-case of thorium are exactly the'same as previously described with beryllium. Extrusion temperatures ranging from 400-600 C. have been found suitable for this metal. Pressures of 26 to 40 tons per square inch have been encountered in extruding 2.980" billets to 1.490 rod (reduction ratio 4:1). At a reduction ratio of 3:1 (1.490 to 0.840") pressures in the range of 20 to 40 tons per square inch were required to bring about the extrusion.

' We have also found that beryllium may be stamped in the temperature range of 300500 C., preferably at about 425 C. Temperatures higher than this could be used if die materials were available. When carrying out this procedure, a blank is placed in a die made of 18 molybdenum-4 tungsten-1 carbon steel having its top and bottom surfaces machined to the contour desired on the top and bottom surfaces of the stamped product. The die and contents are heated to a suitable temperature Within the aforementioned range and a pressure of about tons per square inch is applied to the beryllium in not less than thirty seconds to full load. It is essential that the beryllium be up to the desired temperature before the pressure is applied. Under these conditions, the beryllium undergoes plastic flow and about twenty seconds after the pressure has been applied, it has been forced into the recesses cut in the die faces, filling them completely. When removed from the die, the resulting button is a very accurate replica of the die surfaces and only needs to be cleaned on the edges, which is done by a light grinding.

What is claimed is:

1. A method of deforming beryllium which comprises healing the metal to a temperature between 300 and 500 C. and forcing the metal through a die faced with a lubricant washer selected from the group consisting of graphite aquadag and molybdenum sulfide.

2. A method of extruding beryllium which comprises heating the beryllium to a temperature in the range of 300-500 C. and forcing the beryllium through a die faced with a'graphite washer.

3. A method of extruding beryllium which comprises heating the beryllium to a temperature of 450 C. and forcing the beryllium through a die faced with a graphite washer.

References Cited in the file of this patent UNITED STATES PATENTS 2,630,220 Sejournet Mar. 3, 1953 2,653,494 Creutz Sept. 29, 1953 OTHER REFERENCES MIT-1034, Part I, Technical Progress Report for the Period July Through September 1949, by A. R. Kaufmann, MIT, October 26, 1949. (Page 5 is only part of interest in instant case.) 

1. A METHOD OF DEFORMING BERYLLIUM WHICH COMPRISES HEATING THE METAL TO A TEMPERATURE BETWEEN 300* AND 500*C. AND FORCING THE METAL THROUGH A DIE FACED WITH A LUBRICANT WASHER SELECTED FROM THE GROUP CONSISTING OF GRAPHITE AQUADAG AND MOLYBDENUM SULFIDE.
 2. A METHOD OF EXTRUDING BERYLLIUM WHICH COMPRISES HEATING THE BERYLLIUM TO A TEMPERATURE IN THE RANGE OF 300*-500*C. AND FORCING THE BERYLLIUM THROUGH A DIE FACED WITH A GRAPHITE WASHER. 